1. Field of the Invention
The present invention relates to a white organic light emitting diode (W-OLED) display device and a method of fabricating the same.
2. Description of the Related Art
In recent years, with rising interests in information displays and increasing demands to use portable information media, researches and commercialization of light-weight and thin-profile flat panel displays (FPDs) for substituting traditional displays such as cathode ray tubes (CRTs) have been actively carried out.
In the flat panel display field, light weight and low power consumption liquid crystal display (LCD) devices have received well-deserved attention until the present time, but the liquid crystal display device is not a light emitting device but a light receiving device and also has drawbacks in brightness, contrast ratio, viewing angle, and the like, and thus the development of new display devices has been actively carried out to overcome such drawbacks.
An organic light emitting diode display device among new display devices is a self-emission type, which has excellent characteristics in viewing angle, contrast ratio, and the like compared to the liquid crystal display device. Moreover, the organic light emitting diode device does not require a backlight unit, thereby allowing light-weight and thin-profile as well as having an advantage in the aspect of power consumption. In addition, the organic light emitting diode device may have an advantage of allowing direct current low voltage driving and fast response speed, and particularly have a beneficial advantage in the aspect of fabrication cost.
Contrary to liquid crystal display devices or plasma display panels, the process of fabricating an organic light emitting diode display device may be very simple since only deposition and encapsulation processes are necessary for the fabrication process. Further, if the organic light emitting diode display device is driven with an active matrix scheme having a thin-film transistor, which is a switching element, for each pixel, then it may be possible to obtain the same brightness level even when a low current is applied, thereby having an advantage of low power consumption, fine pitch, and large-sized screen.
Hereinafter, basic structure and operation characteristic of the organic light emitting diode display device will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram for explaining the light emitting principle of a typical organic light emitting diode display device.
A typical organic light emitting diode display device may include an organic light emitting diode as illustrated in FIG. 1. The organic light emitting diode may include organic chemical layers 30a, 30b, 30c, 30d, 30e formed between an anode 18 which is a pixel electrode and a cathode 28 which is a common electrode.
In this case, the organic chemical layers 30a, 30b, 30c, 30d, 30e may include a hole injection layer 30a, a hole transport layer 30b, an emission layer 30c, an electron transport layer 30d, and an electron injection layer 30e. 
If a drive voltage is applied to the anode 18 and cathode 28, then holes passed through the hole transport layer 30b and electrons passed through the electron transport layer 30d move to the emission layer 30c to form excitons, and as a result the emission layer 30c emits visible light.
In the organic light emitting diode display device, pixels made of organic light emitting diodes having the foregoing structure are arranged in a matrix form and such pixels are selectively controlled by using a data voltage and a scan voltage to display an image.
The organic light emitting diode display device can be classified into a passive matrix display device and an active matrix display device using TFT as a switching element. Between them, according to the active matrix scheme, TFT which is an active element is selectively turned on to select a pixel, and the light emission of the pixel is maintained by a voltage maintained at a storage capacitor.
FIG. 2 is an equivalent circuit diagram for one pixel in a typical organic light emitting diode display device, illustrating an equivalent circuit diagram for a typical pixel with 2T1C (including two transistors and one capacitor) in an organic light emitting diode display device using an active matrix scheme.
Referring to FIG. 2, a pixel of the organic light emitting diode display device using an active matrix scheme may include an organic light emitting diode (OLED), a data line (DL) and a gate line (GL) crossed with each other, a switching TFT (SW), a driving TFT (DR) and a storage capacitor (Cst).
Here, the switching TFT (SW) is turned on in response to a scan pulse supplied from the gate line (GL) to turn on a current path between its own source electrode and drain electrode. A data voltage supplied from the data line (DL) during the on-time period of the switching TFT (SW) passes through the source electrode and drain electrode of the switching TFT (SW) and applied to a gate electrode and a storage capacitor (Cst) of the driving TFT (DR).
At this time, the driving TFT (DR) controls a current flowing through the organic light emitting diode (OLED) according to a data voltage applied to its own drain electrode. Further, the storage capacitor (Cst) stores a voltage between the data voltage and the low-level power voltage (Vss), and then constantly maintains for a frame period.
In recent years, getting out of small-sized display panels for portable devices, interests have been concentrated on the medium to large-sized display market, and white organic light emitting diodes (W-OLEDs) have received a lot of attention as a technology for satisfying the market demand. The W-OLEDs may use a color filter for implementing the red, green and blue colors, and also use a planarization layer to compensate a step of the color filter.
FIG. 3 is a cross-sectional view schematically illustrating the structure of a white organic light emitting diode display device.
Referring to FIG. 3, a typical W-OLED display device may implement the red, green and blue colors using a color filter 6G, 6W, 6R, 6B. The color filter 6G, 6W, 6R, 6B may be patterned on the substrate 10, and then a photo acryl material may be used as a planarization layer 15c to compensate a step of the color filter 6G, 6W, 6R, 6B.
In this case, the color filter 6G, 6W, 6R, 6B may be formed with a thickness of about 1-2 μm to implement the color characteristics, and the planarization layer 15c may be formed with a thickness of about 2-3 μm to compensate the step.
An anode 18 may be formed by using indium tin oxide (ITO) subsequent to forming the planarization layer 15c. 
In this case, a bank layer 25 may be formed to be overlapped with the anode 18, but outgas may be generated from the color filter 6G, 6W, 6R, 6B and planarization layer 15c during the degradation process and moved through a boundary surface of the anode 18, thereby generating pixel shrinkage in which an image is shrunk from the pixel edge. The outgas may have an effect on the reliability of the white organic emission layer 30, thus generating pixel shrinkage.
In addition, the planarization layer 15c may serve as a wave guide at an upper portion and a lower end portion of the pixel such that light generated from the white organic emission layer 30 is transmitted to generate fine light beams in a non-emission region.
For example, though vacuum annealing is carried out subsequent to depositing the organic emission layer 30, it is seen that pixel shrinkage occurs after 240 hours has passed at 80° C.
In other words, materials for the color filter 6G, 6W, 6R, 6B used in W-OLED may include dye, pigment, dispersing agent, and the like, but they may be a cause of outgas emission, thereby having an effect on the reliability of the organic emission layer 30 to generate pixel shrinkage.